A major source of Pb exposure and toxicity for wild birds is the ingestion of Pb-based projectiles from ammunition, especially Pb shotgun pellets used in hunting and target shooting, and to a lesser extent Pb-containing bullets. Globally, tens of thousands of tons of metallic Pb projectiles are deposited into the environment every year from hunting and other shooting activities. Although many bird species actively ingest spent Pb shotgun pellets from soils and sediments, this phenomenon has been most extensively studied in waterfowls which frequently ingest these items from the bottoms of lakes, ponds, and marshes, and from fields, mistaking them for food items such as seeds, or for grit (small stones used to help birds grind up food). Once ingested, these pellets often become lodged in the muscular gizzard of waterfowl, where Pb dissolves over time as a result of the grinding action of the gizzard combined with the acidic environment of the upper digestive tract. Dissolved Pb is absorbed into the blood stream and is carried to various organs where it can exert toxic effects. Lead primarily targets the central and peripheral nervous systems, the muscles of the gizzard, the kidneys, and the blood cells, causing loss of coordination, difficulty ingesting food, anemia, emaciation, and ultimately death. Besides waterfowl, numerous other bird species have been documented to have ingested Pb shotgun pellets and bullet fragments from both aquatic (wetland) and terrestrial (upland) habitats.
Prior to North American regulations restricting the use of Pb-based ammunition for migratory bird hunting, it was estimated that 2-3 % of the North American fall waterfowl migration died annually of Pb poisoning from shot ingestion. Since 1991 in the US, and 1996 in Canada, national regulations prohibiting the use of Pb for waterfowl hunting have been in effect, and have led to dramatic declines in the ingestion of Pb and the average concentrations of Pb in waterfowl tissues. A number ofother nations (e.g., Denmark, Great Britain, Finland, the Netherlands, Norway, Sweden) have also established regulations prohibiting the use of
Pb-based ammunition for certain activities, especially the hunting of waterfowl and other wetland shooting. A number of metals and alloys have been tested and approved as nontoxic when ingested as shotgun pellets at realistic rates by birds; approved materials include iron, tin, bismuth, bismuth/tin, tungsten, tungsten/iron, tungsten/nickel/copper, and several tungsten/plastic mixtures.
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